A widely used medium for storing data is magnetic tape. Tape cartridges are one of the most popular formats for storing data on tape. Tape cartridges are usually smaller and less expensive than reel-to-reel tapes. Two widely used tape cartridges are the dual reel cartridge and the single supply reel cartridge. In a dual reel tape cartridge, both the supply reel and the take-up reel are housed in the cartridge. In a single supply reel tape cartridge, the take-up reel is built into the tape drive along with an automatic tape threading mechanism.
When a tape cartridge is not in use in a tape drive or tape player, it is possible for the reel(s) to rotate idly and cause the tape to lose tension and become partially unwound from the reel. To avoid this problem, tape cartridges usually include some type of reel lock. One common type of reel lock uses one or more pawls to engage teeth formed around the outer edge of one of the reel flanges. VHS, Hi-8 and DLT style tape cartridges commonly use this type of lock. En these cartridges, the lock is released as the cartridge is installed in the tape drive or player and, therefore, the tape reel may be unlocked before the tape drive engages the reel. The moment at which the tape drive engages the reel and can control the angular position and velocity of the reel is the most desirable time to release the lock. The IBM 3480 single reel cartridge, for example, uses a locking mechanism with mating coaxial gears formed at the hub of the reel rather than at the outer edge of the reel. The mating gears are separated to unlock the reel as the tape drive engages the reel hub. The lock is released when a post on the rotor of the drive motor presses a small lock button at the center of the reel hub. The lock button is surrounded by a protective shroud to help prevent the inadvertent release of the lock when the cartridge is not installed in the tape drive. The shroud is flush with the reel teeth to minimize the cartridge drop required to engage the reel with the motor drive gear in a low profile tape drive. Although this design is advantageous because the lock is not released until the drive mechanism engages the reel hub, the shroud occupies valuable space that might otherwise be used for the drive motor and its bearings.